Transparent electrodes, useful for a variety of applications such as visual displays and solar cells, can be constructed using many different materials, and have often included indium tin oxide (ITO) films. Factors in the usefulness of such electrodes include cost considerations (e.g., the high price of indium), brittleness of films and the use of high vacuum deposition. Other transparent electrodes are constructed from solution-processed networks of nanostructures such as carbon nanotubes (CNTs), graphene, and silver. However, junction resistances between CNTs and boundry resistances between graphene nanostructures are relatively high. Moreover, the aspect ratios of silver nanowires are often less than ˜100:1, and silver is relatively expensive.
Sheet resistance (Rs) and optical transmittance (T) are two parameters that are of interest for some applications of transparent electrodes. Different types of devices demand different levels of Rs and T. For example, some high performance touch screens stringently require high T (>95%), but tolerate an Rs of 400-600 Ohm/sq. For some solar cells and large area displays, Rs may be set low enough, e.g., less than 20 Ohm/sq, to avoid undesired voltage drops and joule heating during device operation.
Indium tin oxide (ITO) has been widely used as a standard transparent electrode in various types of optoelectronic devices. Due to the constantly increasing demand of ITO for consumer electronics and the low abundance of indium (In), the price of ITO has continually increased throughout the past decade. In addition, the brittle nature of many ITO thin films frustrates their use in flexible applications.
Carbon nanotubes (CNT) and graphene can also be implemented in various applications. However, a sheet resistance of 100-1000 Ohm/sq at 80% optical transmittance in the visible range, achievable in many carbon-based materials, can be insufficient to suit various needs, such as in solar cells. Lithography steps can be difficult and costly to implement on a large scale. In solution processed silver nanowires, the lengths of the nanowires are typically less than 10 μm and the use of a polymer surfactant results in charge transport barriers which limit the conductivity.
These and other issues continue to present challenges to the manufacture and implementation of electrodes in applications using or benefitting from transparency characteristics.